Glycerol Metabolism: Study with Video Lessons, Practice Problems & Examples

Glycerol metabolism primarily focuses on ATP production through glycolysis, with gluconeogenesis serving as a secondary function for energy storage. The process begins with the hydrolysis of triacylglycerol into glycerol and fatty acids. Glycerol is then phosphorylated by glycerol kinase, consuming ATP to form glycerol 3-phosphate, which is oxidized by glycerol 3-phosphate dehydrogenase to produce dihydroxyacetone phosphate (DHAP). DHAP can enter glycolysis or gluconeogenesis, linking lipid metabolism to carbohydrate metabolism.

Glycerol Metabolism Concept 1

Now, when it comes to Glycerol Metabolism, we're going to say the main purpose is ATP production through glycolysis. And we're going to say, a secondary function is energy storage through gluconeogenesis. If we take a look here at Glycerol Metabolism, we have the digestion of our lipids that can break down into Glycerol and fatty acids, but here we're looking at Glycerol itself. Here, Glycerol Metabolism would convert that into DHAP. This DHAP could enter glycolysis and become pyruvate. Now, here, we're not concerned with stage 3 or stage 4 when it comes to food catabolism, so we're going to ignore that. But when it comes to these two, DHAP and pyruvate, they themselves could be routed within stage 2 within the cytosol to create glucose. So just remember, besides the main function of ATP production through glycolysis, where we make pyruvate, which then can go into the citric acid cycle, and then electron transport chain and oxidative phosphorylation for ATP production, we could have them being used through gluconeogenesis to make glucose for storage. Right? So just remember, these are basically the two functions that are possible when it comes to glycerol metabolism.

Glycerol Metabolism Concept 2

In this video, we'll take a look at the stages of Glycerol Metabolism. Glycerol Metabolism begins with the hydrolysis of our triglyceride glycerol molecule into glycerol and 3 fatty acids. The newly released glycerol then travels to the liver to undergo two biochemical reactions. Reaction 1 is irreversible, and it consumes one ATP for energy. In this reaction, we have our glycerol, and by using ATP as a source of inorganic phosphate, we create Glycerol 3-Phosphate. For Reaction 2, it results in the transformation of Glycerol 3-Phosphate into DHAP, which is Dihydroxyacetone Phosphate. In this process, we use NAD⁺, it becomes reduced into NADH. Being reduced means that our Glycerol 3-Phosphate is oxidized to form our DHAP. This gives us an overall view of glycerol metabolism. So, just keep in mind that it's composed of these two reactions: we're going to go from glycerol to DHAP.

Glycerol Metabolism Example 1

Now, here in this example question it says, "Which of the following statements about Glycerol Metabolism is true? A. It involves an oxidative phosphorylation step." Now, remember, glycerol metabolism occurs within the cytosol of food catabolism. This represents stage 2. Oxidative Phosphorylation doesn't happen until step 4. So, this has nothing to do with glycerol metabolism if we're talking about the two steps involved. The first step is energy-producing in nature. Remember, the first step is irreversible and it consumes 1 ATP of energy. Because of this, it's not energy-producing; it's energy-consuming. A source of glycerol is from the TAG molecule that must first undergo hydrolysis. This statement is true. Remember, we're going to start out with our TAG molecule and undergoes hydrolysis to form glycerol and three fatty acids. It's this newly created glycerol molecule that can then undergo metabolism. So, C is a true statement. A hydrolase enzyme can be used in the first step to convert glycerol to Glycerol 3-Phosphate. So, here, hydrolase, we're trying to hydrolyze this enzyme. Remember what's happening? We're going from glycerol to Glycerol 3-Phosphate. And if you've watched my earlier videos on enzymes, we know that if we're going to add an inorganic phosphate to our starting molecule that would involve a kinase enzyme, not a hydrolase enzyme. So, this statement is incorrect. So here, the only statement that's true would have to be option C.

Glycerol Metabolism Concept 3

Now our tag molecule has undergone hydrolysis to create our glycerol molecule. The glycerol molecule then enters stage 1 or reaction 1 of glycerol metabolism. We're going to say here that reaction 1 is a phosphorylation reaction. Here, the enzyme Glycerol Kinase catalyzes the phosphorylation of Glycerol. So here we have Glycerol Kinase as our enzyme being used. Here's our glycerol molecule. We have to use ATP as a source of not only energy but inorganic phosphate. So, ATP will relinquish or give up one of its phosphate groups changing it into ADP and then we're going to say here that that inorganic phosphate group goes to carbon number 3 of the glycerol molecule transforming it into Glycerol 3 Phosphate. Here, we'd write it as PO32-. So this is our newly created Glycerol 3 Phosphate molecule. We're using a kinase, remember, that helps in the transferring of an inorganic phosphate group from one molecule to another. This represents phosphorylation reaction 1 of glycerol metabolism.

Glycerol Metabolism Concept 4

Now, reaction 2 of Glycerol Metabolism represents an oxidation reaction. Remember, when we've talked about oxidation reactions in the past, the class of enzyme we utilized was dehydrogenase. So here we're going to say the dehydrogenase enzyme oxidizes Glycerol 3 Phosphate to Dihydroxyacetone Phosphate, otherwise known as DHAP. Now, here we've utilized 1 NAD⁺, it's going to be reduced to 1 NADH. NADH represents an energetic molecule. If we take a look here, we have glycerol 3 phosphate as our substrate, our beginning material for reaction 2. NAD⁺ gets reduced, so that glycerol 3 phosphate can be oxidized. The enzyme we'd utilize is Glycerol 3 Phosphate dehydrogenase. Remember, the name of the enzyme is just the substrate name followed by the class of enzyme. Again, oxidation reactions we've utilized dehydrogenase as a class of enzymes in previous sections. Now, here dihydroxyacetone, we still have our inorganic phosphate here, and what we have occur is we have a secondary alcohol here in the middle. It gets oxidized to a ketone, giving us DHAP. Now, DHAP should be familiar because we've covered this molecule as well. Recall that DHAP can then directly enter reaction 5 of glycolysis or reaction 8 of gluconeogenesis. Okay? So this would represent the second reaction of glycerol metabolism.

Glycerol Metabolism Example 2

We take a look here at this example question, it says, which of the following statements is true in regard to Glycerol Metabolism? Here, the first step involves the use of a kinase that phosphorylates a secondary alcohol group. Remember, we're changing glycerol to Glycerol 3-phosphate, that alcohol represents not a secondary alcohol, but a primary alcohol. So this would not be true. NAD⁺ represents a high energy molecule produced during step 2. No, it's not NAD⁺ that represents the high energy molecule produced, instead, it's NADH. An isomerase enzyme could be used for the conversion of Glycerol 3-phosphate to DHAP. Now, remember, an isomerase would just create another isomer for glycerol from Glycerol 3-phosphate. That would mean that we have the same number of elements in both compounds. But we're not using an isomerase, we're doing an oxidation, so we're using a dehydrogenase. We're actually losing elements. DHAP and Glycerol 3-phosphate don't have the same molecular formula, therefore, they're not isomers of each other. So an isomerase wouldn't have been used. So this leaves d as our answer. Here, the ATP molecule serves as a phosphate source to phosphorylate a primary alcohol group. That's correct. Remember, we're going from glycerol to Glycerol 3-phosphate, we are adding an inorganic phosphate to a primary carbon. It was a primary alcohol when we've phosphorylated it. So here, the only answer that's correct would have to be option d.

Which of the following outlines the overall pathway of glycerol metabolism?

• A. (1) Phosphorylation (2) Decarboxylation (3) Reduction
• B. (1) Cleavage of a triacylglycerol (2) Phosphorylation (3) Oxidation
• C. (1) Carboxylation (2) Phosphorylation (3) Decarboxylation
• D. (1) Oxidation & Decarboxylation (2) Isomerization (3) Cleavage of a triacylglycerol (4) Hydration

Which of the following represents the complete chemical reaction for the two stages of glycerol metabolism?

• A. Glycerol  +  NAD⁺  +  ADP  → Glycerol-3-phosphate  +  NADH  +  ATP
• B. Glycerol  +  NAD⁺  +  ATP  →  DHAP  +  NADH  +  H⁺  +  ADP
• C. Glycerol-3-phosphate  +  NADH  +  H⁺ →  Glycerol  +  ADP  +  P_i
• D. Fatty acid + NaOH + H₂O →  Glycerol  +  NADH  +  H₂

The glycerol derived from lipolysis of a triglyceride molecule is converted into glycerol-3-phosphate followed by dihydroxyacetone phosphate. Which step does this dihydroxyacetone phosphate enter in terms of glycolysis?

• A. Step 1
• B. Step 3
• C. Step 5
• D. Step 10

Here’s what students ask on this topic:

What is the main purpose of glycerol metabolism?

The main purpose of glycerol metabolism is ATP production through glycolysis. Glycerol is converted into dihydroxyacetone phosphate (DHAP), which enters the glycolytic pathway to produce pyruvate. Pyruvate can then enter the citric acid cycle and the electron transport chain, leading to oxidative phosphorylation and ATP production. Additionally, glycerol metabolism serves a secondary function in energy storage through gluconeogenesis, where DHAP is used to synthesize glucose for storage.

What are the stages of glycerol metabolism?

Glycerol metabolism involves two main stages. The first stage is the phosphorylation of glycerol by glycerol kinase, consuming ATP to form glycerol 3-phosphate. The second stage is the oxidation of glycerol 3-phosphate by glycerol 3-phosphate dehydrogenase, producing dihydroxyacetone phosphate (DHAP). DHAP can then enter glycolysis or gluconeogenesis, linking lipid metabolism to carbohydrate metabolism.

How is glycerol converted into DHAP?

Glycerol is converted into dihydroxyacetone phosphate (DHAP) through two biochemical reactions. First, glycerol is phosphorylated by glycerol kinase, consuming ATP to form glycerol 3-phosphate. Second, glycerol 3-phosphate is oxidized by glycerol 3-phosphate dehydrogenase, using NAD⁺ and producing NADH, to form DHAP. This process links glycerol metabolism to glycolysis and gluconeogenesis.

What enzymes are involved in glycerol metabolism?

Two key enzymes are involved in glycerol metabolism. The first enzyme is glycerol kinase, which catalyzes the phosphorylation of glycerol to form glycerol 3-phosphate, consuming ATP. The second enzyme is glycerol 3-phosphate dehydrogenase, which catalyzes the oxidation of glycerol 3-phosphate to dihydroxyacetone phosphate (DHAP), using NAD⁺ and producing NADH.

What is the role of DHAP in glycerol metabolism?

Dihydroxyacetone phosphate (DHAP) plays a crucial role in glycerol metabolism as an intermediate that links lipid metabolism to carbohydrate metabolism. DHAP can enter the glycolytic pathway to produce pyruvate, which is used for ATP production. Alternatively, DHAP can be used in gluconeogenesis to synthesize glucose for energy storage. This dual role makes DHAP a key molecule in energy production and storage.